Reflective elements are incorporated in traffic signs, pavement markings and apparel. Pavement markings, for example such as those on the centerline and edge of a roadway, provide visual guidance for motor vehicle drivers. The visibility provided by these pavement markings is particularly vital for night time navigation and for navigation during inclement weather conditions.
U.S. Pat. No. 6,127,020 to Bacon, Jr. et al. teaches that such pavement markings typically include glass microspheres that are partially embedded in a binder layer containing reflective pigment particles such as titanium dioxide (TiO2) or lead chromate (PbCrO4). As light from the headlamp of a vehicle impinges upon the microsphere, it is refracted towards the reflective pigment. Refraction as used herein refers to the deflection of light from its original pathway. The light passes through the optical element and is scattered by the pigment-containing pavement paint. A portion of the scattered light is directed back through the optical element and is directed back along the original path towards the driver, increasing the visibility of the markings. This results in a retroreflective effect wherein the most intense light travels back along the illumination axis, which is the centerline between the headlamp and the microsphere, and the light becomes dimmer the farther it is viewed from the illumination axis. Retroreflection as used herein refers to the tendency of light to travel back along its original pathway upon hitting certain surfaces.
The intensity of the light returning to the driver depends upon, among other things, the effective refractive index of the pavement marking. Refractive index as used herein refers to the magnitude by which the speed of light is reduced within a medium. The microspheres have an inherent refractive index; however, U.S. Pat. No. 6,796,740 to Chiron et al. explains that a lower effective refractive index will result if a film of water from recent rainfall has covered the pavement marking. The angle of incidence with which the light impinges upon the microsphere also bears upon the intensity of the light reaching the driver's eyes. Furthermore, retroreflectivity may diminish as traffic erodes the pavement marking surface, if the traffic causes the microspheres to become dislodged from the binder.
In order to maintain retroreflectivity, it has been suggested to use a reflective marking system wherein a first layer of binder is applied to the surface of the substrate, and a first layer of optical elements is partially embedded in the top surface of the first layer of binder. A second layer of binder is applied to cover the first layer of optical elements such that the first layer of optical elements defines a plurality of undulations in the second layer of binder and a second layer of optical elements is partially embedded in the top surface of the second layer of binder and has an exposed-lens surface portion. As the top layer of optical elements and binder is eroded, the underlying layer optical elements will be exposed, thereby maintaining retroreflectivity. Such a system is described in U.S. patent application Ser. No. 12/428,117, filed Apr. 22, 2009, which is incorporated by reference herein in its entirety.
In order to enhance the application of the pavement marking, drying agents are applied to the binder. These drying agents allow the pavement to be marked during damp conditions and also speed up the application process, thereby minimizing the restriction to full utilization of the roadway. The typical application of paint, drying agent and beads is as follows:                1. Paint is applied to the pavement surface;        2. Paint drying accelerant is applied on top of the paint;        3. Anti-skid glass grains are applied on top of the paint; and        4. Retroreflective glass beads are applied and embedded in the paint.        
In use, it is often difficult to adequately intermix the drying agent with the binder in order to achieve optimum results. In addition, the drying agent generally is formulated as beads which have a tendency to coagulate and block the dispensing mechanism. This clogging requires periodic cleaning and causes unscheduled delays in the pavement marking. The clogging also prohibits the accurate metering of the dispensed drying agent.
It is highly desirable to provide a reflective marking system which provides a high degree of retroreflectivity, allows for improved intermixing of the drying agent and binder, and also minimizes the clogging of the drying agent dispenser.